Sync is a great idea. But it doesn't work in a logical way. I've pretty much written it off, yet still find times I have to use it (often with disastrous results. For example, I have a main PC and its the most current as far as bookmarks, logins, etc. I added another PC and used it a while. Then I wanted ALL the "proper" stuff Sync-ed with my new one. What happened was that things move copied and swapped with this other PC, including passwords and the book marks were moved around pell mell. The result was 3 hours trying to undue the damage it did to both machines. I ended up copying the bookmarks over manually (.json files) and had to manually copy passwords and logins after I figured out which were now bad.

It would be great if we had control of the direction that things were copied, i.e. source PC --> target PC

The other issue is that bookmarks are randomly placed. Lots of duplicates and put in weird places. - It would also be nice if we had control over what folders of bookmarks were copied.After one Sync my book marks increased from 1200 to 2300, so I had to figure out what to keep.

Regarding, passwords; there was a nice extension that made a file you can USB over and add to your /security/logins. Now, depending on the version etc. you can manually copy over Key4.db or Key3.db but this isn't backwards compatible with FFesr and is prone to messing up ones profile.

As far as Sync goes, there's developer logic and normal person user logic. I used Sync long before it was made a standard feature in Firefox 4.0; the Weave extension which was used to develop Sync in Firefox 3.5 and 3.6. IMO, overall there are few very experienced users around here or at SUMO who really use Sync any longer; too many bad experiences with Sync losing their data or just being flakey. And now with the "push" to enroll more users of Sync by making the signup for a Firefox Account something that every one is expected to do - message with every new installation and with every update for current users - my feeling is that Mozilla is making a huge mistake to tie Firefox reputation with the bug-iness that is Sync services.

I agree that Sync was a great idea, but Mozilla managed to make too many bad moves or poor feature implementations which caused too many "sore losers of data" and lost too many old time users. I lost "faith" in Sync 4 months before Firefox 4.0 was released back in March 2011 when Mozilla made changes that cut backwards compatibility with the Weave extension, which prevented me from continuing my 2nd device which couldn't run Firefox 4.0 beta at that time. Later on I did find a Linux distro that I could run on that Netbook, but by that time I was "turned off" on Sync due to so many support issues surrounding Mozilla poor implementation of the "Recovery Key" which IMO was too sophisticated for the average user. IOW, mismatched "logic" and users who were too stupid to save that "Key" which was needed to recover their data from Sync. It took Mozilla until Firefox 29 to "fix" that situation, and when they did the "fix" it compounded user issues in a different direction. But long before then I had moved well beyond "testing" upcoming changes being worked on for Firefox (and Sync) and wasn't in a position of trying to reason via Bugzilla with suggestions that made more sense.

As far as directional control with Sync, that just isn't a parameter that Mozilla ever wanted to provide. All devices are equal to Sync with regards to a Master / Slave perspective or "Source" > "Target". Sync is a closed system that was made to run unattended and to be connected to the Sync server whenever Firefox is running. Minimal user control; 2 = on/off or connected / not connected and manual Sync button before the Sync Tabs feature was added and the "manual Sync button" became the Synced Tabs button.

Yes, many users have wished for features that Xmarks (earlier known as Foxmarks before Sync was around and before Xmarks expanded to support Chrome), but from the start of the Weave project that was off the table for consideration; I was following Weave Bug reports during development and saw "wont fix" complaints about lack of directional control often. And as far as controlling Sync for a partial group of bookmarks, with the manner in which Sync works and how Firefox stores bookmarks in places.sqlite - just not possible to do that; and IMO if Mozilla even tried to implement a feature like that they would screw it up.

As far as issues with Sync regarding "misplaced" bookmarks / poor arrangement; sometimes the initial Sync to a new device does take some time (many hours or sometimes days, depending on how the device is being used) and the issue can be exacerbated by the user doing a manual rearrangement to try to "fix" the situation when Sync is enabled on that device AND connected to the Sync server. If the user does do a manual reorganization of their bookmarks, it is important that they Disconnect from the Sync server with that device so that the changes don't get sent to the Sync server piecemeal; when that reorganization is complete, that Connect back to the Sync server so those changes get sent all at once and make damn sure that Firefox on another device isn't running.

And for just over 2 years the use of Sync in Firefox for Android or Firefox for iOS (along with Desktop / Laptop Firefox) has been causing strange issues with bookmarks appearing to "multiply", which IMO is due to corruption in the places.sqlite file on desktop / laptop builds of Firefox. And wow is it a "pain" to fix the corruption completely when a "Firefox for" device is part of the Sync connected devices!

Key3.db to key4.db has an upgrade toggle preference and is the user "went back" to an older version like ESR 52 after the Profile was upgraded to Quantum the older "Passwords" wouldn't appear, from what I have heard. I haven't actually seen that, I created a new Profile for my Firefox Quantum installations to start from scratch. Past experiences with massive changes updates of Firefox has told me to start with a fresh Profile; the time spent doing that is worth the downside of having to waste a lot more time fixing "things that just happen".

A mind is a terrible thing to waste. Mine has wandered off and I'm out looking for it.